Conventionally, in the case of, for example, a spark ignition engine, a structure has been known in which multipoint ignition electrodes (hereinafter referred to as “peripheral electrodes”) are embedded into a head gasket which is disposed between a cylinder head and a cylinder block in order to obtain larger flames than only using a single ignition gap (see, for example, Patent Document 1).
In such a structure, flames spread both concentrically and three-dimensionally in a combustion chamber from a central ignition plug arranged at the center of a ceiling of the combustion chamber and a plurality of peripheral electrodes. Accordingly, the time required for the flames to spread over the entire space of the combustion chamber is shortened, resulting in less time-loss and approaching a theoretical cycle, so that thermal efficiency can be improved. Thereby, quick combustion of an air-fuel mixture can be realized.
However, since the peripheral electrodes have a structure where the peripheral electrodes protrude toward the combustion chamber, it is difficult to secure endurance reliability. That is, since the electrodes of ignition plugs are generally consumed, the electrodes have to be replaced for each certain period. In the case of the multipoint peripheral electrodes, the plurality of peripheral electrodes are electrically connected in series with each other by a conductive member. Hence, even when only one of the peripheral electrodes is consumed, there are drawbacks that all of the peripheral electrodes have to be replaced for each head gasket.
Further, while the peripheral electrodes disclosed in Patent Document 1 are held by ceramics and are fired, the sintering of ceramics is difficult to be carried out. That is, it is difficult to sinter and fabricate the ceramics without hollows at a state where the metallic peripheral electrode is exposed, so that commercialization and mass production thereof cannot be easily achieved.
Further, there is also a problem that it is difficult to realize the commercialization because the electrodes are embedded into the head gasket.